1. Field of the Invention
The invention pertains to a combination double bottom block of a block and tackle arrangement in which two pulley blocks, each with several small pulleys, are connected to each other by a rocker, and where a load hook is supported on the rocker.
2. Description of the Related Art
Depending on the various combinations of working loads and boom lengths, crane operators have been forced until now to use a double bottom block in certain working load situations.
In the case of short boom systems, however, the operator can also work with a single hoist winch.
In the case of conventional bottom blocks, conversion of the conventional double bottom block leads to a halving of the working load which can be lifted.
If the operator wants to lift working loads of more than 50% of the nominal working load on the boom, he must use an additional, larger single bottom block or work with a double bottom block.
Double bottom blocks, however, require two pulley blocks, which are a certain distance apart and connected to each other by a rocker.
When two hoisting cables are being used, this rocker with the double bottom blocks makes it possible for balancing operations to occur within the hook block.
These pulley blocks can be converted to a half-size single bottom block by shifting one of the pulley blocks to the center of the bottom block and by removing the other pulley block.
As a result, however, the nominal working load is cut by 50%, e.g., from 320 tons to 160 tons.
If the two pulley blocks are locked to allow the use of a single hoisting cable, the gap remaining between the pulley blocks must be bridged.
This usually leads to large angles of deflection, which can be reduced only by increasing the distance between the pulley block on the crane and the bottom block.
This leads to an undesirable reduction in the possible lifting height.
Furthermore, with each receiving arrangement care must be taken to ensure that the moments are in equilibrium around the center axis of the block.
It has been found repeatedly that it is highly favorable for a small pulley to be located in the geometric center of the block.
This behaves neutrally with respect to the moment relationships of the receiving.
The two pulley blocks of a double bottom block thus almost always have an odd number of small pulleys.
If the double bottom block is now locked, what is created out of the two pulley blocks, each with, for example, 5 small pulleys, is one large single block with an unfavorable 10 small pulleys and a gap in the geometric center of this hook block.
The task of the invention is to eliminate this problem.